Geophysical surveys provide a cross-sectional map of the geology below the surface of the earth. Using sound waves, the shape and character of the geology may be revealed to indicate pockets within the sedimentary layers where oil and gas may be trapped, indicating locations where exploratory wells may be drilled for further geological analysis. Geophysical studies between bore holes or between bore holes and the ground surface have been done in the past using small charges of explosive impulsive sources and vibratory sound sources, but none of these types of sound sources have proved to be either practical or robust enough to be used over extended periods of time.